Poking at a nanotube with a pointy rod can
lead to intriguing nano-science.

Nanotubes, tiny carbon pipes, may someday play
a pivotal role in bringing nanoscale technology
into everyday use. Now researchers have
discovered that prodding these tubes with a
pointy tip can alter their ability to carry an
electric current.

The results, the first to demonstrate how
mechanical deformations can affect a molecular
wire's electrical properties, were published in
the June 15 issue of the journal Nature by
Hongjie D,ai Stanford assistant professor of
chemistry, and graduate student Thomas Tobler in
collaboration with University of Kentucky
theoretical physicists.

The discovery could be used for making tiny
electromechanical devices, such as transducers
that convert mechanical movements into electrical
signals. Other applications include creating
high-frequency telephone lines to carry voice and
data and making on/off switches for nanoscale
computer chips.

Dai and his colleagues studied carbon
nanotubes that are less than one-millionth the
diameter of a human hair and just millionths of
an inch long. Each tiny structure resembles a
rolled-up graphitic sheet of carbon atoms
arranged in a honeycomb pattern.

To prod the nanotube, the researchers used the
sharp tip of an atomic force microscope (AFM). A
real-space microscopy technique, the AFM makes
images of surface topography by dragging a pointy
tip over a structure's bumps and folds. The tip
reads the shape like a blind person reads
Braille. The textures are then translated into a
visual image.

To conduct experiments on a single nanotube,
Dai's group used a technique he perfected with
AFM co-inventor Calvin Quate , professor of
electrical engineering at Stanford, and graduate
student Jing Kong. They placed an array of finely
powdered metal nanoparticles on a silicon dioxide
substrate, and then fed a carbon-containing gas
(methane) over the substrate heated to a high
temperature. The carbon infused into the metal
particles, which acted as catalysts that
converted carbon atoms into honeycomb-lattice
nanotubes.

The researchers used the technique to grow a
single nanotube across a silicon dioxide trench.
They then attached an electrode to each end of
the tube. They used the AFM tip to push the wire
down into the trench, while measuring the wire's
electrical conductance.

The group was initially surprised to observe
that the flow of electricity dropped sharply as
the nanotube bent. When the AFM tip was removed,
the tube straightened and the flow of electricity
returned to normal. Previous theoretical studies
predicted no significant change in the
conductance of nanotubes due to mechanical
deformation.

Dai hypothesized that a dent that formed near
the AFM tip could be responsible for strongly
affecting the electrical flow. To make sense of
the results, Dai enlisted the help of
theoretician Shi-yu Wu at the University of
Kentucky. Wu and his colleagues used computer
simulations to show that the AFM tip dented one
wall toward the other, as when a garden hose gets
kinked and the flow of water is stopped.

As one side of the tube is pushed closer to
the other, carbon atoms form bonds across the
inside of the tube. Normally, each carbon atom
binds to three other carbons, leaving one
electron free for use in conducting electricity.
But when the walls of the tube come close
together, each carbon binds to four rather than
three carbons. The resulting decrease in the
number of free electrons causes the electrical
conductance to slow.

"The AFM tip squashes the tube, causing
each atom to bond with more atoms," said
Dai. "This causes the tube to turn from an
electrical conductor into an insulating structure
similar to that found in diamonds."
Remarkably, the dent disappears once the
perturbing tip is removed. This high mechanical
reversibility allows the full recovery of the
nanotube's electrical property, Dai said.

"Dai's work is a very exciting
experimental demonstration of what our
theoretical work predicted," said Kyeongjae
Cho, Stanford assistant professor of mechanical
engineering, "namely that local nanotube
deformation is a way to develop different
functional components of nanotube
transistors."